Elevators are equipped with automatic braking devices acting as safety gears designed to stop the elevator if its speed rises too much. Conventionally, the braking device used is a gripper type brake that grips a guide rail. Generally, the safety gear is used to stop downward movement. Such a gripping brake can also be used to stop the elevator for other reasons, e.g. in a case where an error in operation results in the elevator leaving a door zone with doors open. The basic structure of a conventional brake gripping a guide rail, a safety gear, is as follows. The safety gear frame contains a hollow with a braking surface facing towards a guide rail placed in the hollow. Placed in the hollow is also a wedge or a roller, which, mounted on a track in the hollow, is driven against the elevator guide rail when the safety gear is activated. The elevator guide rail is placed between the braking surface and the wedge or roller. The track is so shaped that, when the wedge or roller moves on the track in the direction of the guide rail, the guide rail is pressed by the wedge or roller against the braking surface, thus producing a braking action that stops the elevator car. The safety gear can also be mounted on the counterweight.
The compression applied by the safety gear to the guide rail is set by means of a spring. Via the friction between the braking surface and the guide rail, the magnitude of the compressive force determines the deceleration of the elevator. The use of spring-loaded compression compensates for the variations in compressive force occurring during the deceleration. In the course of time, several different technical solutions for an elevator safety gear have been developed. A commonly used safety gear type is a large U-shaped spring made of spring steel, with a wedge which is driven between the ends of the spring when the safety gear is activated. Safety gears of this type are also often provided with a release wedge for easier disengagement from the guide rail. However, the U-shape of such a safety gear opens during the gripping action, which is why the braking surface has only partial contact with the guide rail. Further drawbacks with this type of safety gear are its large size and high price. Safety gears have also been developed in which the safety gear frame is relatively rigid and the springiness of the compression applied by the safety gear to the guide rail is achieved using separate springs. However, these safety gears comprise a large number of separate parts and they are more complex in composition. Such safety gears are lighter than earlier safety gear types, but like these, they are quite expensive. As the length of the safety gear is a significant factor affecting the size, operation, price and placement of the safety gear, in order to reduce the safety gear length, a roller track with a changing gradient has been used in some safety gears employing a roller to engage the guide rail. The purpose of the steeper gradient at the beginning of a track with changing gradient is to quickly eliminate the clearance between the guide rail and the friction surface of the safety gear in the initial stage of safety gear operation. The gradient of the first part of the track cannot be increased very much because a steeper track would result in jeopardising the ascent of the roller to the part of the track that produces the actual braking force or, to ensure the ascent of the roller to the part of the track that produces the braking force, in a necessity to move the roller using a larger force from the overspeed governor. However, the force obtained from the overspeed governor cannot be increased very much because even the regulations relating to elevator structure and safety impose certain upper and lower limits on the force generated by the overspeed governor for the triggering of safety gears.